Spanish dancer

Short definition from Wiktionary

Noun

1. 1
   A species of nudibranch sea slug, Hexabranchus sanguineus, commonly growing up to a maximum length of 25 cm, with some reports to 40 cm in the Red Sea.
2. 2
   A species of deep-sea sea cucumber, Enypniastes eximia, with webbed swimming fin-like structures at the front and back of its body.